1. Field of the Invention
The present invention relates to a technology for suppressing electromagnetic interference (EMI) resulting from an electromagnetic wave emitted from an electronic device, and particularly, to a technology for suppressing the electromagnetic interference by reducing the electromagnetic wave emitted from the electronic device by supplying a frequency-spread clock signal to a controller such as a CPU in the electronic device.
2. Description of the Related Art
Heretofore, the International Special Committee on Radio Interference (CISPR), the Voluntary Control Council for Interference by Information Technology Equipment (VCCI), and a like organization have regulated and set an allowable upper limit of an electromagnetic wave emitted from information processing apparatuses, telecommunication devices, electronic office appliances, and the like (hereinafter, called as “electronic devices”) to suppress electromagnetic interference (EMI). In view of this, generally, the electronic devices are provided with an electromagnetic shield, ferrite beads, a choke coil, and the like to reduce an electromagnetic wave emitted therefrom.
Also, there is known a technique (see e.g. Japanese Unexamined Patent Publication No. 2002-33858), as one of the measures to suppress the EMI by reducing an electromagnetic wave emitted from the electronic devices. According to the technique, a spread spectrum clock generator (hereinafter, called as “SSCG”) is used to supply a frequency-spread clock signal to a controller in the electronic device to keep the electromagnetic wave emitted from the electronic device from exceeding the predetermined allowable upper limit. The SSCG spreads the frequency of a clock signal generated by a clock signal generator such as a crystal oscillator having a crystal unit based on a predetermined constant spread ratio to supply the frequency-spread clock signal to the controller in the electronic device.
The clock signal is adapted to determine a computation processing speed of a CPU or a like device in the controller of the electronic device or the like. The computation processing speed may be lowered due to the clock signal whose frequency has been spread by the SSCG. A certain degree of lowering of the computation processing speed may be tolerated to suppress the EMI. However, it is desirable to minimize the lowering of the computation processing speed in the electronic device.
Since the spread ratio in the conventional SSCG is set to a constant value, as mentioned above, a clock signal whose frequency has been spread based on the constant spread ratio is constantly supplied to the electronic device irrespective of the intensity of an electromagnetic wave emitted from the electronic device. Therefore, even if the electromagnetic wave emitted from the electronic device is temporarily reduced depending on an operated state of the electronic device, an environment where the electronic device is installed, or other factor, the SSCG unduly spreads the frequency of the clock signal. In such a case, the computation processing speed of the CPU or the like in the controller of the electronic device which is operated in synchronism with the frequency-spread clock signal, the operation speed of the electronic device, or the like may be unduly lowered by the amount corresponding to the excessively spread of the frequency of the clock signal.